There's no such rule, in fact, #2 in the "Sanctioning" says that the tourney might require Premium. Because of the game-load and the size of this tournament it wouldn't run as smoothly if there are going to be groups that are only playing half their games at a time because they have to wait for the free player to finish up their games. And it's unreasonable to expect that a free player will devote their entire allotment to just one tournament when they could play in a tournament that only needs one game spot free.

Updated to this point. We now have 93 players, that makes us over 2/3 of the way full.